1. Field of the Invention
This invention relates to a power supply device for a camera of the kind using an interchangeable lens or the like and more particularly to a power supply device to be used for a power supply to an accessory such as an interchangeable lens as well as for a power supply to a camera body.
2. Description of the Related Art
As a result of the recent advancement of camera automation, cameras are equipped with an automatic exposure control device, automatic focusing device, an automatic light amount control device, etc. For these devices, various electronic circuits, a motor and a battery are arranged within the camera. In the case of lens interchangeable type cameras which are provided with these devices, they have motors and electronic circuits arranged not only on the side of their camera bodies but also on the side of their lenses. Therefore, the camera of this kind is provided with a connector which is disposed at mount parts for coupling the camera body and the lens with each other and has electric contacts or connection terminals arranged to electrically connect various devices disposed within the camera body to those disposed within he lens.
These connection terminals include terminals for receiving a power supply and terminals for signal exchange. The power supply terminals includes terminals of two different kinds, one for receiving a positive power supply and the other for receiving a negative power supply. With respect to the power supply on the negative side, some electric connection part that is other than an electronic contact, such as the mount, can be employed. As for the power supply on the positive side, two power supply channels are generally arranged, one for operating an actuator such as the motor and the other for operating electronic circuits for control and information exchanges. Terminals are discretely arranged for these two channels. However, in the event of inadequate contact between contact points or during the process of mounting the lens on the camera body, the power supply is either not received or only intermittently received by the control circuits while the actuators are receiving the power supply. Under such a condition, a faulty action might be performed with the motor driven by the power supply. In other words, while the motor is rendered operative by the power supply, the control circuits are not receiving any power supply. Then, a normal operation of the motor is hardly expectable because of the unreliability of control signals.
Further, in cases where the motor power supply is alone effected while the communication lines of the electronic circuit on the side of the camera body is not completely in touch with the communication lines on the side of the lens due to inadequate contact between contact points or during the lens mounting process, the electronic control circuit on the side of the lens fails to correctly receive information from the camera body. As a result, an instruction for forward rotation of the motor might be mistaken for reverse rotation; or an instruction for a halt might be unreceived. A normal operation is not assurable also under such a condition. A conductive foreign matter such as conductive dust or water sticking to a communication line causes a short-circuit state between the communication line and the mount part or between adjacent connection terminals. In that case, the electronic control circuit on the side of the lens also fails to correctly receive information from the camera body. Then, a normal operation is likewise not assurable.
In the conventional lens interchangeable type camera, the number of connection terminals is reduced by using a ground line and its connection terminal for the motor on the side of the lens in common with a ground line and its connection terminal for the electronic control circuit which is also on the lens side. The common use of the ground line and the connection terminal, however, tends to break both the electronic circuits disposed on the sides of the camera body and the lens due to a difference in ground potential between the lens and the camera body. For example, assuming that the ground potential of the electronic circuit on the side of the camera body is at zero volt, that of the electronic circuit on the side of the lens is one volt (with 1 A of ground current flowing at contact resistance 1 .noteq. of the terminal), when a signal of a high level of 5 V and a low level of 0 V is applied from the electronic circuit on the camera body side to the electronic circuit on the lens side, the electronic circuit on the lens side is receiving a voltage which is lower than the ground potential by 1 V. This would cause either a faulty action or thermal breakage of the electronic circuit on the lens side. To solve this problem, an improved interchangeable lens has been proposed as disclosed in U.S. Pat. application Ser. No. 012,862, filed Feb. 10, 1987, assigned to the assignee of the present invention. The interchangeable lens disclosed in the previous application, however, has been found to have the probability of making some erroneous connection during the process of mounting the interchangeable lens on a camera body, because. The connection terminals on the sides of both the camera body and the lens have flat contact faces and are arranged to protrude to the same extent and to have the same pitch between them.